Dorsal is a maternal regulatory factor that initiates dorsoventral patterning of the precellular Drosophila embryo. It is a member of the Rel family of transcription factors that is initially distributed throughout the cytoplasm of unfertilized eggs, but shortly after fertilization it translocates into nuclei. This nuclear transport process is regulated by the Toll signaling pathway, which is employed in a variety of processes, including insect immunity and acute/inflammatory responses in mammals. The resulting Dorsal nuclear gradient specifies the embryonic mesoderm, neurogenic ectoderm, and dorsal ectoderm, through the differential regulation of target genes such as snail, sog, and dpp. The snail gene encodes a zinc finger repressor that establishes the boundary between the presumptive mesoderm and neurogenic ectoderm. It is a short-range repressor that must bind within 100 bp of either upstream activators or the core promoter in order to inhibit gene expression. Dpp is a member of the TGF-beta family, and a Dpp activity gradient subdivides the dorsal ectoderm into the amnioserosa and dorsal epidermis. The Dpp gradient depends on sog, which encodes a secreted inhibitory protein that binds Dpp. High levels of Sog are thought to inhibit Dpp signaling, while low levels enhance signaling. The proposed study represents a continuation of our efforts to determine how the Toll-Dorsal signaling pathway controls the dorsoventral patterning of the early Drosophila embryo. The research plan includes three specific aims: (i) determine how Snail functions as a repressor; (ii) identify the components of the Toll signaling pathway that diffuse in precellular embryos to create the Dorsal nuclear gradient; and, (iii) determine how Sog- Dpp interactions create a peak Dpp signaling threshold that specifies the amnioserosa.